Dental restorations such as crowns or artificial teeth have traditionally been made by firing porcelain on a cast body of precious metal such as an alloy of gold. The physical properties of these precious alloys are well understood in dentistry, and the alloys bond properly with porcelain and are compatible for use in the mouth. Gold alloys are easy to melt and cast, are sufficiently ductile to permit burnishing of casting margins in the finishing of dental restorations, and can be polished to a high luster to resist plaque formation. Alloys of precious metals, however, are relatively heavy, and have increased in cost to such an extent that substitute materials have been sought in recent years.
It is now known that certain stainless alloys of non-precious metals can be used in dentistry, and examples of specific nickel alloys and processing techniques are set forth in U.S. Pat. Nos. 3,716,418, 3,727,299, 3,749,570 and 3,761,728, the disclosures of these patents being incorporated herein by reference. These nickel alloys have a higher modulus of elasticity than that of precious metal alloys, contributing to better sag resistance of the ceramo-metal structure after repeated firings in a furnace.
The higher strength of nickel alloys enables use of thinner castings which minimize reduction of the natural tooth structure in preparation for installation of the restoration. Nickel-alloy restorations are also light in weight and low in thermal conductivity, and these features provide greater comfort to a patient with a sensitive or deeply involved tooth. These alloys bond satisfactorily to porcelain, and further have the important economic advantage of being significantly lower in cast than gold or other precious-metal alloys.
There are various shortcomings of known nickel alloys. For example, these alloys are difficult to finish and polish, thereby requiring more dental laboratory time as compared to precious-metal alloys. Bond strengths of nickel alloys to dental porcelains are sensitive to the necessary repeated firings in laboratory manipulation and preparation, and this factor can affect clinical performance of the porcelain and nickel-alloy system. It is desirable, therefore, for a nickel alloy to have a high porcelain-metal bond strength to be compatible with the laboratory processing involved in making a restoration. Another problem is that slags of known nickel alloys tend to adhere to clay crucibles used to melt the alloy prior to casting. It takes time and effort to grind or chip off these tenacious slags to avoid possible contamination of other alloys during subsequent casting.
It is believed that these and other problems arise from the use in known alloys of elements such as beryllium, tin, silicon, gallium and boron which are added for improved melting and casting performance. In contrast to precious-alloy ingots which melt into a pool with little or no slag, prior-art nickel-alloy ingots tend to form into an individual molten mass covered by a thick slag when melted by a torch. This problem is at least partially controlled by use of the aforementioned elements, but not without incurring other problems.
For example, beryllium poses a health risk if not carefully handled during alloy processing. Alloys containing significant amounts of silicon and gallium tend to be brittle, and have an as-cast elongation of only about 2% due to the formation of intermetallic compounds. Alloys of this type must be heat treated at 1800.degree. F. for about 30 minutes, followed by slow cooling in air, to provide sufficient ductility for margin burnishing, and the increased labor cost arising from this processing tends to cancel the reduced cost of a nonprecious alloy. Some other alloys exhibit satisfactory ductility (over 5% elongation as cast), but microscopic carbides and intermetallics in the alloy result in more difficult and time-consuming shaping and polishing as compared to castings of precious alloys.
The new alloy of this invention overcomes these disadvantages of known nickel alloys, while maintaining the advantages of these materials as described above.